Lasers generate optical energy over a broad bandwidth and are capable of lasing at a variety of different wavelengths. For example, the gain medium of an infrared semiconductor laser can emit optical energy within the infrared spectrum that consists of approximately 50 nanometers (nm). In applications requiring laser emissions at a single specified wavelength within the available spectrum, such as in wavelength division multiplexed applications, it is necessary to tune or fix the laser wavelength.
Tuning a laser can be accomplished by filtering the broad bandwidth optical energy generated by a gain medium to isolate a desired wavelength band and directing the isolated wavelength band into a laser cavity. Introducing the desired wavelength band into the laser cavity causes the optical energy to oscillate within the laser cavity at the desired wavelength. The oscillating optical energy is released from the laser cavity as laser light at the desired wavelength. Consequently, the tuning of a laser can be accomplished by controlling the wavelength of optical energy that is fed back into the laser cavity.
Two examples of wavelength-selectable lasers are disclosed in U.S. Pat. No. 4,914,665 to Sorin, which is assigned to the assignee of the present invention, and U.S. Pat. No. 4,955,028 to Alferness et al. (hereinafter Alferness). Referring to FIG. 1, the wavelength-selectable laser has a gain medium 10 which transmits optical energy through an optical fiber 12 to a single external reflector 14, such as a grating. The optical energy in the optical fiber contacts the lone external grating through an exposed region of the optical fiber and is reflected back to the gain medium to generate lasing at the desired wavelength band. The wavelength band of optical energy that the grating reflects back to the gain medium is dependent upon the spacing and orientation of grating ridges. In both Sorin and Alferness, the spacing and orientation of grating ridges are adjusted by hand through mechanical wavelength controller 16 that includes bulky metal frames and adjustment screws.
As an alternative to using an external grating in a wavelength-selectable laser, fiber Bragg gratings can be used to reflect a desired wavelength band of optical energy. A tunable fiber Bragg grating is a thermally or mechanically adjusted grating that is formed completely inside an optical fiber. Wavelength-selectable lasers are known in which a single fiber Bragg grating is substituted for the external grating employed in the Sorin and Alferness lasers. The main disadvantage of a wavelength-selectable laser employing a single fiber Bragg grating is that the fiber Bragg grating is adjustable only over a limited range of wavelengths. For example, a typical fiber Bragg grating can only be temperature tuned over a wavelength range of approximately 0.8 nm, limiting the adjustability of a laser to the same wavelength range.
In lightwave communication systems, lasers are often used as the optical energy source for carrying optical signals. Optical channels recognized by the International Telecommunications Union span a wavelength range of 30 nm. A wavelength-selectable laser that is adjustable only over a range of 0.8 nm does not provide optimal flexibility in many wavelength division multiplexed applications.
Adding additional fiber Bragg gratings into the optical fiber of a laser system provides a wider range of wavelength adjustability in a laser. A disadvantage of adding additional fiber Bragg gratings is that a fiber Bragg grating constantly reflects optical energy at some wavelength band. Since the gain medium of a laser system generates broadband optical energy, each additional fiber Bragg grating will reflect a wavelength band of optical energy back to the gain medium, causing interference and preventing the laser from lasing at a single wavelength in a stable manner.
What is needed is a wavelength-selectable laser that offers the advantages provided by fiber Bragg gratings, while allowing wavelength adjustability over a wavelength range that is practical for use, particularly in wavelength division multiplexed applications such as lightwave communications.